The American scientist Professor Martin Kamen, who has died aged 89, was the co-discoverer of the radioactive isotope carbon-14. The finding transformed biochemistry as a tracer following chemical processes in plants, while its use in the carbon dating of fossils and ancient artefacts between 500 and 500,00 years old revolutionised archaeology.

Kamen's discoveries included confirmation of the idea that all oxygen released in photosynthesis comes from water and not carbon dioxide. He studied the part played by molybdenum in nitrogen fixation, a life-sustaining process which converts atmospheric nitrogen to inorganic and organic compounds for plants and animals; the role of iron in the activity of porphyrin compounds in plants and animals; and calcium exchange in cancerous tumours. Much of this was achieved under extreme stress.

Kamen suffered shamefully during the anti-Communist witchhunts of the 1940s and 1950s. He recorded his despair in his autobiography, Radiant Science, Dark Politics (1985). His troubles began during the wartime Manhattan atomic bomb project, when in 1943 he was assigned to research at the Oak Ridge National Laboratory, Tennessee. He asked a colleague who ran the cyclotron machine to prepare radioactive sodium for an experiment. When Kamen opened the container of sodium, he was surprised that it was glowing purple, signifying a much more intensely radioactive batch than could be produced in a cyclotron.

He deduced immediately that the sodium must have been irradiated in an atomic reactor elsewhere in the laboratory. Because of security, he not been told of its existence. In his excitement, he blurted out his belief to colleagues. Shortly after, an investigation was launched to find out who had leaked the information to him.

When he returned to his regular work at the radiation laboratory of the University of California, Berkeley, Kamen met two Russian officials at a party given by his close friend, the violinist Isaac Stern. Kamen had toyed with a musical career, and accompanied Stern as a viola-player in social evenings of chamber music.

One Russian asked how he might find help to get experimental radiation treatment for a colleague with leukaemia. Kamen made inquiries, and in appreciation the official invited him for dinner at Bernstein's Fish Grotto. Since the Oak Ridge incident, FBI agents had kept Kamen under surveillance. They observed the dinner, and Kamen was sacked shortly afterwards.

During the next decade he fought recurring rumours and accusations that he had leaked atomic bomb secrets. At first, he found all academic positions closed to him, and even worked for a while as an inspector at a shipyard.

After the House Un-American Activities Committee summoned him to testify in 1948, the State Department refused to issue him a passport. In 1951 the Chicago Tribune named him as a suspected spy. He attempted suicide, and it took him 10 years to establish conclusively that the US Army had blacklisted him as a security risk. In 1955, he won a libel suit against the Tribune, and the State Department relented and supplied a passport.

Kamen was the son of Russian émigrés, and born in Toronto. He graduated in chemistry from the University of Chicago in 1933 and took a PhD for research in physical chemistry. One of his mentors, David Gans, suggested he should apply for a research post in chemistry and nuclear physics under Ernest Lawrence at the radiation laboratory in Berkeley. He worked there without pay for six months before Lawrence offered him the job of overseeing the preparation and distribution of the cyclotron's products.

He collaborated with another young chemist, Samuel Ruben, on the discovery of carbon-14 in 1940. They were fascinated by chemical reactions in photosynthesis, which they hoped to unwind by tracing the behaviour of the carbon-based molecules. Most carbon atoms in nature are in the stable form of carbon-12. The idea was to replace some stable carbon-12 atoms with radioactive versions that could always be located by the radiation they emitted. The only known radioactive isotope, carbon-11, had too short a life. In a search for longer-lived isotopes, they bombarded graphite in the cyclotron. The result was carbon-14, which had a 5,730 year half-life. In 1949, chemist Willard Libby used it to invent radiocarbon dating.

Following his dismissal from Berkeley in 1945, Kamen was befriended by Arthur Holly Compton, the Nobel prizewinning physicist, and recruited to run the cyclotron programme in the medical school of Washington University at St Louis. Kamen taught the faculty how to use radioactive tracer materials for research, and his own interests gradually shifted away from nuclear physics and radiochemistry and into biochemistry.

In 1957 he moved to Brandeis University in Massachusetts, and in 1961 he joined the University of California, San Diego, chemistry department, as founding father of the new campus, and began the next significant phase of his research, exploring the mechanisms of photosynthesis in bacteria. He became emeritus professor in 1978 and continued research into the comparative biochemistry of large proteins called cytochromes, and their vital part in photosynthesis for storing energy and in metabolism for converting food into energy.

He received many academic awards. In 1996, he shared the Enrico Fermi Award, which recognised lifetime achievements in energy research, given by the US Department of Energy, (which ran the laboratory that had fired him). The irony was not lost on him, nor did it improve his view of government bureaucrats.

His first marriage, to Esther, ended in divorce in 1943. His second wife, Beka, died in 1963, and his third Virginia, died in 1987. A son survives.

· Martin David Kamen, scientist, born August 27 1913; died August 31 2002.

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